Performance analysis of copper and brass tools in micro-hole drilling of SuperNi 276 using electrical discharge drilling

Abstract

SuperNi 276 is highly valued for its exceptional corrosion resistance and high-temperature performance, making it a critical material in the aerospace and power generation industries. However, fabricating precise micro-holes in this alloy poses challenges with conventional machining techniques. This study explores the use of electro discharge drilling (EDD) to produce high-quality micro-holes in SuperNi 276. Output parameters, including tool wear rate (TWR) and material removal rate (MRR), were measured by varying input factors such as pulse on time, pulse off time, and peak current, using tool diameters 0.5 mm, 0.6 mm, 0.7 mm, and 0.8 mm while maintaining a constant voltage of 2 V. The elemental composition of the recast layer and heat-affected zone (HAZ) was analyzed with energy-dispersive X-ray spectroscopy (EDS). Copper tools in EDD of SuperNi 276 achieve a higher maximum MRR of 0.115938 mm³/s but show significant tool wear with a TWR of up to 0.7391 mm/s. Brass tools, however, exhibit minimal TWR at 0.0026 mm/s, making them ideal for applications requiring longer tool life. Though the maximum MRR for brass is moderate at 0.065940 mm³/s, it offers a balance between efficient machining and reduced wear. Post-EDD, the thickness of the recast layer and HAZ were analyzed. Experimental results indicate that the copper tool exhibits a thinner recast layer and HAZ due to its high thermal conductivity. Microhardness testing showed that brass tools increase the hardness of the recast layer and minimize the HAZ due to superior heat dissipation, resulting in improved material integrity. This study also shows that brass tools maintain a composition similar to the base material, especially in the HAZ, with 58.0% Ni, showing optimal results, and balanced Cr, Mo, W, and Fe levels. Brass tools, under specific configurations, ensure consistent machining and material integrity with minimal wear.

Keywords

SuperNi 276 electro discharge drilling (EDD)energy-dispersive X-ray spectroscopy (EDS)recast layer heat-affected zone (HAZ)material removal rate (MRR)tool wear rate (TWR)

Introduction

SuperNi 276, a nickel-molybdenum-chromium superalloy, is renowned for its exceptional corrosion resistance and mechanical strength in extreme environments. These properties make it an ideal choice for demanding applications such as turbine blades, chemical processing equipment, and other high-stress engineering components. The alloy’s ability to withstand harsh conditions and maintain stability contributes to its widespread use in aerospace, power generation, and environmental engineering sectors.

Micro drilling, a specialized machining technique, is essential for creating extremely small holes ranging from micrometers to millimeters in diameter.¹ This precision process is crucial in fields such as aerospace, electronics, and biomedical devices, where accuracy and miniaturization are vital. Electro discharge drilling (EDD) is a non-traditional method that utilizes electrical sparks to erode conductive, hard-to-machine materials.² The process generates localized heat through electrical discharges between the tool and workpiece in a dielectric fluid, ensuring high precision and allowing for the production of intricate geometries not possible with conventional methods.³ Drilling micro-holes in SuperNi 276 using EDD requires careful optimization of input parameters to achieve desired outputs such as tool wear rate (TWR), material removal rate (MRR), and machining time.

Numerous studies have focused on refining EDD techniques for superalloys and composites. Mishra et al.⁴ experimentally and numerically examined micro-EDM drilling on Inconel-718, focusing on the effects of plasma flushing efficiency on white layer thickness and heat-affected zone (HAZ) depth. Their findings underscore that optimized pulse parameters can reduce surface damage. Jain et al.⁵ optimized pulse electrochemical jet drilling on Incoloy 800, reducing radial overcut and taper. Kumar et al.⁶ developed a self-flushing tool electrode for EDD on Ti6Al4V, enhancing depth and MRR. Kumar and Kumar⁷ investigated electrical discharge micro drilling for ZM21 Mg alloy, producing porous architectures for tissue engineering. Mao et al.⁸ explored assisted EDD and powder-mixed EDM for hard-to-machine materials, improving MRR and tool wear. Machno⁹ examined the machinability of Inconel 718 during electrical discharge drilling, analyzing the effects of five key process parameters on hole quality and tool wear. The study highlights that combining higher current amplitude and extended pulse time enhances hole accuracy and aspect ratio. Singh et al.¹⁰ used a Taguchi L16 array to optimize micro-EDM, finding capacitance critical for performance. Machno¹¹ studied EDD in Inconel 718, emphasizing debris removal for quality. Dhaker and Modi¹² developed a data-driven quadratic regression model to predict and optimize recast layer thickness during electric discharge drilling of Inconel-718. Vasudevan et al.¹³ examined drilling small holes in YSZ-coated superalloys, finding that abrasive water jet machining maintained material integrity best. Other research, such as Doan et al.¹⁴ optimized EDD parameters, achieving significant MRR improvements, while Harane and Unune¹⁵ identified the tool geometry’s impact on performance. Pant and Bharti¹⁶ focused on EDM for micro-holes in Nimonic 80A10, and Sawant et al.¹⁷ found WCu electrodes provided high MRR in µEDD. Kumar et al.¹⁸ highlighted peak currents for minimal recast layers12, and Ansari et al.¹⁹ improved EDD on magnesium nanocomposites using Taguchi and ANOVA. Liu et al.²⁰ investigated abrasive water flow polishing to improve the surface integrity of superalloy micro-holes drilled by EDD, focusing on recast layer removal and surface roughness reduction. Bassoli et al.²¹ investigated the influence of electrode size and geometry on the EDD of Inconel 718, emphasizing the critical role of gap pollution and debris accumulation in discharge ignition. Datta et al.²² conducted a comprehensive machinability analysis of Inconel 601, 625, 718, and 825 during EDM, using a Taguchi based approach integrated with satisfaction and distance functions to determine optimal parameter settings. Their study revealed significant variation in MRRs and surface roughness across alloys, with carbon migration and carbide formation impacting surface integrity due to dielectric decomposition. Dutta and Sarma²³ performed multi-objective optimization of µ-EDM parameters for drilling micro-holes in Hastelloy C-276 using response surface methodology and a multi-objective genetic algorithm. Their findings showed capacitance as the most influential factor, with MOGA outperforming RSM in achieving higher MRRs, lower tool wear, and minimal diametral overcut.

Previous studies have predominantly focused on alloys such as Inconel 718, Incoloy 800, Ti6Al4V, Nimonic 80 A, and magnesium-based alloys. These works have addressed improvements in flushing efficiency, optimization of pulse parameters, tool design, and the use of assisted or powder-mixed EDM techniques to enhance MRR, minimize tool wear, and improve hole quality. Other studies have concentrated on reducing defects such as recast layer thickness, taper, and micro-cracks, or on improving post-machining surface integrity. Despite these advancements, comparatively fewer investigations have been carried out on SuperNi 276 (Hastelloy C-276), a superalloy with unique corrosion resistance and strength in extreme environments. The machining challenges posed by this alloy, such as rapid tool wear, debris accumulation, and recast layer formation, are not fully understood in the context of EDD. Moreover, while parameter optimization has been attempted for other superalloys, there is limited research that systematically explores the combined influence of current, pulse on/off time, tool diameter, and tool material specifically for SuperNi 276.

Nevertheless, existing studies suggest that voltage has minimal impact on output parameters,^24–31 such as MRR, TWR, and machining time. Hence, in this work, voltage is kept constant while varying current, pulse on/off time, tool diameter, and tool material. An L16 orthogonal array, designed using the Taguchi method, is employed to minimize the number of experiments. TWR, MRR, microhardness, and the elemental composition in the recast layer and HAZ are being analyzed using energy-dispersive X-ray spectroscopy (EDS).

Methodology

Material selection

SuperNi 276

MIDHANI Ltd., Hyderabad, supplied SuperNi 276, which was utilized in the production of steam turbine blades for power generation applications. Table 1 provides the major alloying elements along with their respective weight percentages.

Table 1.

Composition of SuperNi 276.

Major constituent element	Weight (%)
Nickel (Ni)	58.2
Chromium (Cr)	16.2
Molybdenum (Mo)	16.1
Tungsten (W)	3.1
Iron (Fe)	4.2

EDD machine and tool material

The EDD machining experiments were carried out using copper and brass tubular tools with 0.5 mm, 0.6 mm, 0.7 mm, and 0.8 mm diameters for machining Super Ni 276²⁰ as per ASTM B643 standards

Control parameters

The control parameters for EDD experimentation include peak current (I), pulse on time (Ton), pulse off time (Toff), and tool diameter (D), while voltage is kept constant at 2 V. These factors are pivotal in determining drilling performance and output quality. The selected parameter ranges are as follows: tool diameter (0.5–0.8 mm), pulse on time (10–40 µs), pulse off time (2–5 µs), and peak current (2–5 A).

Estimation of output parameters

MRR and TWR are measured as output parameters while machining. Post-machining, the recast layer and HAZ form due to heat generation, leading to changes in microhardness and elemental composition. The thickness of these zones is examined using an Olympus BX53M microscope, while the changes in constituent elements are analyzed through EDS using the ZEISS SmartEDX system (Sigma 300, Germany). To assess variations in hardness, the Vickers microhardness test (Mitutoyo HM-200, Japan) is conducted in the recast layer zone and the HAZ. Furthermore, changes in the chemical composition of Super Ni 276 after EDD with copper and brass tools may affect the base material, introducing unwanted elements into the recast layer and HAZ, which could influence the material’s mechanical properties. The photography of the equipment used in the present study is shown in Figure 1.

Experimental investigation

Estimation of MRR and TWR

To study the impact of control parameters on output parameters, 256 experiments would be required. To optimize this number, the four-factorial Taguchi method is applied. Consequently, an L16 orthogonal array is used in this work. The levels and values for Taguchi’s L16 orthogonal array are presented in Table 2. MRR and TWR are estimated based on the design of experiments for both copper and brass tools.

Table 2.

Taguchi’s L16 orthogonal array with values of levels.

Experiment number	Tool diameter (mm)	Pulse on time (µs)	Pulse off time (µs)	Current (A)
1	0.5	10	2	2
2	0.5	20	3	3
3	0.5	30	4	4
4	0.5	40	5	5
5	0.6	10	3	4
6	0.6	20	2	5
7	0.6	30	5	2
8	0.6	40	4	3
9	0.7	10	4	5
10	0.7	20	5	4
11	0.7	30	2	3
12	0.7	40	3	2
13	0.8	10	5	3
14	0.8	20	4	2
15	0.8	30	3	5
16	0.8	40	2	4

Recast layer thickness and HAZ

Drilling of micro-holes generates significant heat during the process. The dissipation of this heat occurs through the thermal conductivity of the tool and workpiece, with the majority being removed via the coolant and dielectric medium. However, a portion of the heat remains within the workpiece, leading to material grain restructuring due to resolidification, which results in the formation of a recast layer. Additionally, heat transfer around the drilled hole creates a HAZ.³² Analyzing the thickness of these zones provides valuable insights into the structural integrity and mechanical properties of the material. An electron microscope is employed to measure the thickness of these zones at three different locations around the drilled hole. The average of these measurements is then calculated to ensure accurate and reliable results.

Analysis of microhardness

Microhardness is the measure of a material’s resistance to indentation when tested on a microscopic scale with small loads. After EDD, tests are conducted using a Vickers micro hardness tester on the base material, recast layer zone, and HAZ.

Analysis of changes in elemental composition

The changes in composition of base material in the recast layer zone and HAZ were analyzed using EDS according to ASTM E1508−12 a (2019) standard, based on weight percentage. EDS works on the principle of X-ray fluorescence, where an electron beam excites a sample, causing the emission of characteristic X-rays that reveal the sample’s elemental composition. The system includes an X-ray detector, a pulse processor, and analysis software that produces an energy spectrum with peaks corresponding to different elements, enabling both qualitative and quantitative analysis.³³

Results and discussions

MRR and TWR

Figure 2 shows the specimens after EDD with copper and brass tools. Analyzing the experimental data from Table 3, which details the process parameters for micro-hole machining using copper and brass tools on EDD, the focus is on maximizing MRR while minimizing TWR. The MRR and TWR are estimated using equations (1) and (2):

Table 3.

MRR and TWR of SuperNi 276 for copper and brass tools.

Experiment number	Copper tool		Brass tool
Experiment number	TWR (mm/s)	MRR (mm³/s)	TWR (mm/s)	MRR (mm³/s)
1	0.0171	0.002872	0.0478	0.005120
2	0.0562	0.006615	0.1386	0.011658
3	0.1132	0.007406	0.1548	0.014018
4	0.1333	0.008722	0.1940	0.017575
5	0.3438	0.052988	0.3529	0.049871
6	0.7391	0.073722	0.1111	0.015700
7	0.1585	0.020678	0.0303	0.006423
8	0.5000	0.070650	0.2692	0.032608
9	0.3333	0.054950	0.1558	0.029973
10	0.3333	0.059177	0.3415	0.056290
11	0.2750	0.057698	0.4286	0.065940
12	0.0359	0.010349	0.0680	0.015700
13	0.0750	0.025120	0.0625	0.018840
14	0.0367	0.010048	0.0026	0.007709
15	0.4516	0.097239	0.0870	0.032765
16	0.5000	0.115938	0.1078	0.029553

TWR= \frac{Initial length of the tool-Final length of the tool}{Time taken for machining}

(1)

MRR= \frac{Volume of the material removed}{Time taken for machining}

(2)

In the case of the copper tool, the minimum TWR of 0.0171 mm/s was obtained in experiment 1, while the maximum MRR of 0.115938 mm³/s occurred in experiment 16. For the brass tool, the lowest TWR was 0.0026 mm/s in experiment 14, which indicates its superiority in reducing tool wear. Its highest MRR, 0.065940 mm³/s, was recorded in experiment 11. Although this value is lower than the peak MRR of the copper tool, it remains significant.

Brass proved to be more suitable for operations where tool life is critical, as it consistently exhibited a much lower TWR. Copper offered higher MRR, but at the expense of greater tool wear. Thus, brass provides a better compromise by combining acceptable MRR with excellent tool wear resistance. The comparative variations of MRR and TWR for both tools are illustrated in Figures 3 and 4.²³

Estimation of recast layer thickness and HAZ

The microscopic image of the recast layer and HAZ from experiment 6 is presented in Figure 5 to estimate the thickness of the recast layer and HAZ. Similar measurements were performed for all drilled holes, and the results are summarized in Table 4, which provides a detailed analysis of the recast layer thickness observed during micro-hole drilling using copper and brass tools, as specified in the DOE.

Table 4.

Recast layer and HAZ of copper and brass tool as per DOE.

Experiment number	Recast layer thickness (µm)		HAZ (µm)
Experiment number	Copper tool	Brass tool	Copper tool	Brass tool
1	6.39	18.82	68.67	79
2	30.06	33.61	103.38	134.15
3	21.93	23.41	92.55	110.4
4	16.53	26.19	108.14	116.26
5	18.62	36.04	143.31	89.23
6	19.46	68.15	111.02	91.61
7	8.27	14.87	121.64	147.96
8	38.54	14.32	104.31	160.21
9	15.10	66.17	164.63	134.86
10	33.70	21.81	119.76	134.89
11	10.71	23.06	99.56	180.98
12	23.48	31.88	81.96	129.53
13	18.67	172.99	130.86	172.24
14	23.40	47.88	65.03	132.15
15	23.57	42.72	42.86	121.85
16	21.49	117.87	43.64	151.96

To identify the best experimental conditions, it is essential to evaluate the results based on the minimal recast layer and HAZ. These factors are critical as they directly impact the structural integrity and mechanical properties of the material. For the copper tool, the recast layer thickness is lowest in experiment 1, measuring 6.39 µm, which is significantly smaller than other experimental values. The HAZ in this experiment is also relatively small, at 68.67 µm, making it a suitable choice for minimizing material alteration. While experiment 15 achieves the lowest HAZ (42.86 µm), its recast layer thickness (23.57 µm) is higher than in experiment 1. Thus, experiment 1 provides an optimal balance, offering the least recast layer thickness and a reasonable HAZ. For the brass tool, the recast layer thickness is lowest in experiment 8, measuring 14.32 µm. However, the HAZ for this experiment is relatively high, at 160.21 µm, which might compromise the material’s structural integrity. Experiment 15, on the other hand, provides a more balanced outcome, with a recast layer thickness of 42.72 µm and a moderate HAZ of 121.85 µm. This makes experiment 15 a better overall choice for the brass tool when considering both parameters.

The recast layer directly affects surface roughness and potential rework requirements, whereas the HAZ impacts the material’s strength and mechanical properties.³⁴ Prioritizing a balance between these factors ensures that the material remains functional and structurally sound. Thus, experiment 1 for the copper tool and experiment 15 for the brass tool are selected as the best experiments for achieving minimal thermal damage while preserving the desired material properties.

Investigation of microhardness

The measured microhardness values in the base material, recast layer, and HAZ are shown in Table 5.

Table 5.

Microhardness at various zones.

Experiment number	Copper tool			Brass tool
Experiment number	Base material	Recast layer zone	HAZ	Base material	Recast layer zone	HAZ
1	343.2	347.4	319.6	347.2	334.6	320.6
2	351.9	345.9	324.6	345.9	340.1	323.5
3	348.6	354.3	317.3	350.6	341.7	311.6
4	341.8	347.6	326.4	342.6	334.3	317.7
5	345.3	368.1	325.2	342.4	350.6	338.4
6	351.1	342.6	316.4	348.2	352.6	315.9
7	344.3	341.6	326.2	346.6	345.2	329.2
8	348.6	335.3	331.6	342.8	356.6	326.6
9	341.9	328.8	316.5	348.1	344.8	317.9
10	346.1	334.3	324.1	342.6	348.1	324.1
11	341.4	327.6	319.6	349.4	356.4	336.6
12	343.2	335.1	314.4	343.8	345.1	321.3
13	344.2	327.6	307.1	346.4	355.9	314.1
14	344.8	338.3	313.8	349.1	357.6	329.8
15	345.6	326.9	314.6	342.5	338.2	317.7
16	337.6	345.1	324.2	343.8	334.7	326.9

Base material

From the experiment results, the base material has shown stable microhardness values, irrespective of the electrode used. With copper, hardness values varied between 337.6 HV and 351.9 HV, while with brass, they ranged from 342.4 HV to 349.4 HV. The results suggest that the EDD process had little effect on the base material, since heat and material removal were confined to the immediate machining zone, thereby maintaining the original hardness of the base material.³⁵

Recast layer zone

The recast layer has the maximum hardness among the studied regions. Using copper electrodes, hardness values were observed between 327.6 HV and 368.1 HV, whereas with brass electrodes, the range was 334.3–357.6 HV. The higher hardness in this layer is attributed to the quick solidification of molten material during machining, producing a brittle structure with refined grains and possible alloying from the electrode. Brass generally produced slightly greater hardness compared to copper, due to its higher thermal conductivity, which aided rapid heat removal and the development of a fine-grain structure.

Heat-Affected Zone (HAZ)

Hardness values in the HAZ were lower than in the recast layer and base material. For copper, the values ranged from 307.1 HV to 331.6 HV, while for brass, they ranged between 314.1 HV and 336.6 HV. The reduction in hardness due to thermal softening and grain coarsening. Brass showed slightly higher values than copper in this zone, which is due to its superior heat conductivity.

When comparing both tool materials, brass generally resulted in higher hardness values in the recast layer as well as in the HAZ. This is due to better heat dissipation, which minimized softening in the HAZ and promoted the formation of fine-grain structures in the recast layer. The variations in microhardness for copper and brass tools are shown in Figures 6 and 7.

Changes in constituting elements

The changes in the elemental composition of SuperNi276 after EDD were examined using EDS. Initially, EDS was performed on the base material to establish the elemental composition of SuperNi276. Post-EDD, experiments 1 and 16 (created using a copper tool) and experiments 11 and 14 (created using a brass tool) were selected based on optimal output parameters (TWR and MRR). Figure 3 shows the EDS spot on the base material. An amp time of 0.96 µs and a resolution of 130.2 eV were considered for the analysis. Figure 4 depicts the elemental spectrum of base material SuperNi276. According to the EDS results, the primary elements in the base material are Ni (58.2%), Cr (16.2%), Mo (16.1%), W (3.1%), and Fe (4.2%), along with trace amounts of Al, Si, Ti, Mn, V, and other minor elements.

The EDS spot analysis and elemental spectra for experiments 1 and 16 (conducted using a copper tool) in the recast layer zone and HAZ, as well as those for experiments 11 and 14 (conducted using a brass tool) in the same regions, are presented in Figures 8–16. The changes in elemental composition (weight %) in the base material and in the recast layer and HAZ of holes drilled using copper and brass tools are summarized in Table 6.

Table 6.

Elemental composition (weight %) of base material, recast layer zone, and HAZ for copper and brass tools.

Name of the element (w%)			Ni	Cr	Mo	W	Fe
Base material			58.2	16.2	16.1	3.1	4.2
Copper tool	Experiment 1	Recast layer zone	54.2	15.7	16.6	2.9	5.1
	Experiment 1	HAZ	56.1	16.2	16.3	3.1	4.7
	Experiment 16	Recast layer zone	55.6	16.9	16.1	3.4	4.3
	Experiment 16	HAZ	56.2	17.1	13.8	2.76	4.8
Brass tool	Experiment 11	Recast layer zone	57.2	15.1	14.8	2.6	5.1
	Experiment 11	HAZ	56.4	16.8	15.7	2.9	5.0
	Experiment 14	Recast layer zone	56.3	17.7	14.7	2.1	5.3
	Experiment 14	HAZ	58.0	16.5	14.6	2.7	5.1

Table 6 provides the elemental composition (in weight percentage) of the base material, as well as the recast layer zone and HAZ after EDD using copper and brass tools.³⁶ The analysis focuses on five key elements: Ni, Cr, Mo, W, and Fe. The base material initially consists of 58.2% Ni, 16.2% Cr, 16.1% Mo, 3.1% W, and 4.2% Fe. When examining the copper tool experiments, experiment 1 shows the recast layer zone with 54.2% Ni, 15.7% Cr, 16.6% Mo, 2.9% W, and 5.1% Fe, while the HAZ has 56.1% Ni, 16.2% Cr, 16.3% Mo, 3.1% W, and 4.7% Fe. Experiment 16 with the copper tool records slightly higher values in the recast layer and HAZ, including 55.6% Ni and 56.2% Ni, respectively. For the brass tool, experiment 11 features a recast layer zone of 57.2% Ni, 15.1% Cr, 14.8% Mo, 2.6% W, and 5.1% Fe, with the HAZ displaying 56.4% Ni, 16.8% Cr, 15.7% Mo, 2.9% W, and 5.0% Fe. Notably, experiment 14 with the brass tool yields the most consistent results, showing 56.3% Ni in the recast layer and 58.0% Ni in the HAZ, along with balanced concentrations of other elements such as Cr, Mo, W, and Fe.

Considering the analysis, experiment 14 using the brass tool stands out as the optimal choice. It closely aligns with the base material composition, particularly in the HAZ, with 58.0% Ni and minimal deviation in other elements. This suggests that experiment 14 provides better process control and preserves the base material’s properties more effectively, making it the recommended experiment for maintaining material integrity during EDD.

Conclusions

The experimental study on EDD of SuperNi 276 using copper and brass tools reveals significant insights into tool performance, MRR, and TWR. The investigation shows that while copper tools achieve a higher maximum MRR, they exhibit greater tool wear. In contrast, the brass tool has a superior ability to minimize TWR, underscoring its advantage in applications requiring tool longevity. The highest MRR for the brass tool is moderate, balancing effective machining with reduced wear. Experiment 1 (copper tool) and experiment 14 (brass tool) are identified as optimal for minimizing thermal effects while maintaining material functionality. This balance ensures reduced rework and preserved structural integrity. The elemental composition analysis post-EDD shows that the brass tool maintains the closest alignment with the base material, particularly in the HAZ. The study also highlights that tool material significantly influences microhardness distribution, with brass tools enhancing recast layer hardness and minimizing HAZ softening due to superior heat dissipation. Experiment 14 with the brass tool is notably optimal, presenting 58.0% Ni in the HAZ, nearly identical to the base composition and balanced concentrations of Cr, Mo, W, and Fe. This suggests that brass tools, especially in configurations of tool diameter 0.8 mm, pulse on time 20 µs, pulse off time 4 µs, and current 2 A with a constant voltage of 2 V, provide more consistent results with minimal deviation, preserving the integrity of the material. For processes requiring both efficient material removal and minimal tool degradation, brass emerges as the preferred tool due to its balance of moderate MRR and significantly lower TWR.

Authors’ contribution

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by N. Saidulu, P. Laxminarayana, and K. Buschaiah. The first draft of the manuscript was written by N. Saidulu, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data and code availability

Not Applicable.

Footnotes

Declaration of Conflict of Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical approval

The present work does not involve any human samples, tissues, or biological samples.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Saidulu Nakarkanti

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